1. Field of the Invention
The present invention relates to a wound, high voltage radio noise resistant cable, and more particularly to a wound, high voltage radio noise resistant cable having excellent durability and noise prevention characteristics in the 30-200 MHz frequency range, and which is adapted principally for use in the ignition circuits of gasoline engines for automobiles.
2. Description of the Prior Art
It is commonly known that the ignition circuits of gasoline engines for automobiles tend to generate radio noise which can disrupt television and radio signals and sometimes even cause malfunctions to occur within other electical circuits or elements of the automobile. In order to alleviate this problem, the ignition circuit is generally provided with an ignition cable having properties that allow it to prevent such radio noise from occurring.
In the prior art of ignition cables used for the above-mentioned purpose, two types are generally employed. The first type of cable is constructed so as to have a centralized resistor which acts as a bulk resistor, and the second type of cable is constructed so as to have a resistor formed from a non-conductive fibrous material impregnated with a carbon-based powder.
However, for the first type of cable mentioned above, the centralized resistor generates a series resonance from 100 MHz to 200 MHz due to the equivalent capacitance of the centralized resistor and the inductance of the high voltage wire, and this results in markedly reduced radio noise suppression. Similarly, the second type of cable mentioned is insufficient at suppressing radio noise because there is a diminution in impedance due to the equivalent capacitance of the resistor.
Due to the drawbacks of the two types of cables described above, a cable having superior radio noise prevention was proposed as shown in FIG. 1. Namely, the cable 1 comprises a core 2 made from glass fiber or the like which is covered with ferrite rubber 3. Wound around the ferrite rubber covering 3 in the axial direction of the cable is a metallic resistance wire 4, which is in turn covered with an insulator 7. Provided over the insulator 7 is a braided reinforcing which is further covered with a sheath 9.
In the structure of the cable shown in FIG. 1 as described above, the ferrite rubber covering 3 is formed by the addition of ferrite powder to a base polymer. Therefore, the radio noise suppression capacity of the cable depends on such factors as the permeability of the ferrite powder, the thickness of the ferrite rubber covering 3, and the amount of the ferrite added to the base polymer. Consequently, in order to obtain a sufficient radio noise suppression effect, a thick layer of ferrite rubber containing a large amount of high permeable ferrite powder must be provided over the core 2 of the cable. As a result, production costs become quite high and there is a loss of general utility. In addition, since the rubber covering 3 has to be made relatively thick, the overall weight of the cable is increased while the operability is reduced due to a large voltage drop arising from an increase in the electrostatic capacitance of the wire.
In response to the shortcomings of the cable shown in FIG. 1, U.S. Pat. No. 4,435,692 disclosed a wound, high voltage cable having a low electrostatic capacitance. As described in this disclosure, the cable is constructed with a polyaramide fiber reinforcing core having a diameter of less than 1.3 mm which, by virtue its small diameter, allows ferrite rubber covering to be formed to a desired thickness without drastically enlarging the entire diameter of the cable.
Unfortunately, however, even though the diameter of the reinforcing core of the cable disclosed in U.S. Pat. No. 4,435,692 is reduced in comparison to the reinforcing core of the cable shown in FIG. 1, the thickness and composition of the ferrite rubber covering for both these cables remains essentially the same. Therefore, even though there is a difference in the overall diameters for these two cables, the cable disclosed in U.S. Pat. No. 4,435,692 does not provide sufficient improvement of radio noise suppression characteristics.
Accordingly, it has been an ongoing task in this field to try to make a high voltage cable capable of satisfactorily suppressing radio noise, particularly radio noise in the range of 30-200 MHz, even when the thickness of the ferrite rubber covering the reinforcing core is reduced.